For the operation of modern internal combustion engines and compliance with strict emission limiting values, an engine controller determines the air mass which is enclosed in a cylinder per working cycle. As a function of the air mass and the desired “lambda” ratio between the air quantity and fuel quantity, a specific quantity of fuel is injected via an injection valve which is also referred to as an injector in this document. For this purpose, a corresponding fuel quantity setpoint value (MFF_SP) is calculated by the engine controller. The fuel quantity which is to be injected can therefore be dimensioned in such a way that a value for lambda which is optimum for the exhaust gas post-treatment in the catalytic converter is available.
The main requirement made of the injection valve is not only leakproofness to prevent undesired discharge of fuel and preparation of a fuel jet but also chronologically precise dimensioning of the injection quantity. For example in the case of supercharged spark ignition engines which operate with direct injection of fuel, a very high degree of quantity spreading of the required fuel quantity is necessary. For example, for supercharged operation a maximum fuel quantity MFF_max has to be metered per working cycle, while during operation which is close to idling a minimum fuel quantity MFF_min has to be metered. The two characteristic variables MFF_max and MFF_min define here the limits of the linear working range of the injection valve. This means that for these injection quantities there is a linear relationship between the injection time (electrical actuation period (Ti)) and the injected fuel quantity per working cycle (MFF).
For direct injection valves with a coil drive, the quantity spread, i.e. the quotient between MFF_max and MFF_min, can be between 6 and 40 depending on the respective engine power. In specific cases, the quantity spread can be even larger. For future engines with reduced CO2 emissions, the cubic capacity will be reduced and the engine rated power is at least maintained by means of engine supercharging mechanisms. The requirement made of the maximum fuel quantity MFF_max therefore corresponds at least to the requirements of an induction engine with a relatively large cubic capacity. However, the minimum fuel quantity MFF_min is determined by means of operation near to idling and the minimum air mass in overrun conditions of the engine with a reduced cubic capacity, and said minimum fuel quantity MFF_min is therefore decreased. This results in an increased requirement in terms both of the quantity spread and of the minimum fuel quantity MFF_min for future engines.
In known injection systems, a significant deviation of the actual injection quantity from the nominal injection quantity occurs in the case of injection quantities which are smaller than MFF_min. This deviation is due essentially to fabrication tolerances at the injection as well as to tolerances of the output stage, which actuates the injector, in the engine controller and therefore to deviations from the nominal actuation current profile.
The characteristic curve of an injection valve defines the relationship between the injected fuel quantity MFF and the time period Ti of the electrical actuation (MFF=f(Ti)). The inversion of this relationship Ti=g(MFF_SP) is used in the engine controller to convert the setpoint fuel quantity (MFF_SP) into the necessary injection time. The influencing variables, such as the fuel pressure, cylinder internal pressure during the injection process and possible variations of the supply voltage, which are additionally included in this calculation are omitted here for the sake of simplification.
FIG. 7a shows the characteristic curve of a direct injection valve. In this context, the injected fuel quantity MFF is plotted as a function of the time period Ti of the electrical actuation. In a good approximation, a linear working range is obtained for the time periods Ti longer than Ti_min, and the injected fuel quantity MFF is directly proportional to the time period Ti of the electrical actuation. Linear behavior does not occur for time periods Ti shorter than Ti_min. In the illustrated example, Ti_min is approximately 0.5 ms.
The gradient of the characteristic curve in the linear working range corresponds to the static flow through the injection valve during the complete valve stroke. The cause of the non-linear behavior for time periods Ti shorter than approximately 0.5 ms or for fuel quantities MFF<MFF_min is, in particular, the inertia of an injection spring mass system and the chronological behavior during the build up and reduction of the magnetic field through a coil, which magnetic field activates the valve needle of the injection valve. As a result of these dynamic effects, the entire valve stroke is no longer reached in what is referred to as the ballistic region. This means that the valve is closed again before the end position which defines the maximum valve stroke has been reached.
In order to provide a reproducible injection quantity, injection valves are usually operated in the linear working range. A stable operation in the non-linear range is currently not possible since a significant systematic error occurs in the injection quantity owing (a) to the above-mentioned tolerances in the supply voltage and therefore also in the current profile and (b) to mechanical tolerances of injection valves (for example by tensile force of the closing spring, internal friction in the armature/needle system). For reliable operation of an injection valve, this results in a minimum fuel quantity MFF_min per injection pulse, which minimum fuel quantity MFF_min at least has to be provided in order to be able to implement the desired injection quantity precisely in terms of the quantity. In the example illustrated in FIG. 7a, this minimum fuel quantity MFF_min is somewhat smaller than 5 mg.
The electrical actuation of an injection valve usually takes place by means of current-controlled full bridge output stages of the engine controller. A full bridge output stage makes it possible to apply an on-board power system voltage of the motor vehicle, and alternatively a boosting voltage, to the injection valve. The boosting voltage is frequently also referred to as a boost voltage (U_boost) and can be, for example, approximately 60 v.
FIG. 7b shows a typical current actuation profile I (thick unbroken line) for a direct injection valve with a coil drive. FIG. 7b also shows the corresponding voltage U (thin continuous line) which is applied to the direct injection valve. The actuation is divided into the following phases:
A) Pre-charge phase: during this phase with the duration t_pch, the battery voltage U_bat, which corresponds to the voltage of the on-board power system of the motor vehicle, is applied to the coil drive of the injection valve. When current setpoint value I_pch is reached, the battery voltage U_bat is switched off by a two-point regulator, and after a further current threshold has been undershot, U_bat is switched on again.
B) Boost phase: here, the output stage applies the boosting voltage U_boost to the coil drive until a predefined maximum current I_peak is reached. The rapid build-up of current speeds up the opening of the injection valve. After I_peak has been reached, there follows a free-wheeling phase up to the expiry of t_1, during which free-wheeling phase the battery voltage U_bat is then applied to the coil drive. The time period Ti of the electrical actuation is measured from the start of the boost phase. The transition to the free-wheeling phase is triggered by I_peak being exceeded.
C) Commutation phase: the commutation phase begins with the switching off of the voltage, as a result of which a self-induction voltage is generated. Said voltage is limited essentially to the boosting voltage U_boost. The limitation of the voltage during this self-induction is composed of the sum of U_boost as well as the forward voltages of a recuperation diode and of what is referred to as a free-wheeling diode. The sum of these voltages is referred to below as the recuperation voltage. On account of the differential voltage measurement, on which FIG. 5 is based, the recuperation voltage is illustrated in a negative form in the commutation phase.
The recuperation voltage results in a flow of current through the coil, which flow reduces the magnetic field to a minimum. The commutation phase, which depends on the battery voltage U_bat and on the duration t_1 of the boost phase, ends after the expiry of a further time period t_2.
D) Holding phase: here, the setpoint value for the holding current setpoint value I_hold is adjusted using the battery voltage U_bat by means of a two-point regulator.
E) Switch-off phase: switching off the voltage results, in turn, in a self-induction voltage which is also limited to the recuperation voltage. This results in a flow of current through the coil, which flow then reduces the magnetic field. After the recuperation voltage, which is illustrated in a negative form here, has been exceeded, no current flows any more. This state is also referred to as “open coil”. Owing to the ohmic resistances of the magnetic material, the eddicurrents which are induced during the field reduction of the coil decay. The reduction in the eddicurrents leads in turn to a change in the field of the solenoid and therefore to a voltage induction. This induction effect leads to the voltage value at the injector rising to zero starting from the level of the recuperation voltage in accordance with the profile of an exponential function. After the reduction of the magnetic force the injector closes by means of the spring force and the hydraulic force caused by the fuel pressure.
The described actuation of the injection valve has the disadvantage that the precise time of closing of the injection valve or of the injector in the “open coil” phase cannot be determined. Since a variation of the injection quantity correlates with the resulting variation of the closing time, the absence of this information, for example at very small injection quantities which are less than MFF_min, results in a considerable degree of uncertainty regarding the fuel quantity which is actually injected into the combustion chamber of a motor vehicle engine.
DE 38 43 138 A1 discloses a method for controlling and sensing the movement of an armature of an electromagnetic switching element. During the switching off of the switching element, a magnetic field is induced in its exciter winding, which magnetic field is changed by the armature movement. The changes in timing of the voltage applied to the exciter winding which are due to this can be used to sense the end of the armature movement. DE 10 2006 035 225 A1 discloses an electromagnetic actuating device which has a coil. By evaluating induced voltage signals, which are caused by external mechanical influences, the actual movement of the actuating device can be analyzed.
DE 198 34 405 A1 discloses a method for estimating a needle stroke of a solenoid valve. During the movement of the valve needle relative to a coil of the solenoid valve, the voltages which are induced in the coil are sensed and placed in relationship with the stroke of a valve needle by means of a computing model. In order to determine the contact time, the time derivative dU/dt of the coil voltage can be used since this signal has large jumps at the reversal point of the needle movement or armature movement.
DE 103 56 858 B4 discloses an operating method for an actuator of an injection valve. A measured time profile of an electrical operating variable of the actuator is compared with a stored reference curve which represents the chronological profile of this operating variable in a reference pattern.